Lipid is the primary fuel for the beating heart. In fact, glucose utilization is inhibited by oxidation of fatty acids in heart. In exercise studies, it has been suggested that endogenous triacylglycerols (TG) are quantitatively unimportant as a metabolic fuel because of the small absolute decrease in TG content seen after work. In these studies, TG synthesis was not measured. This is an important factor because turnover rate of the intracellular TG pool in muscle is rapid. Therefore, TG synthesis and hydrolysis will be measured in hearts of exercise-trained rats. In the second study, the cAMP system appears to regulate TG lipolysis in muscle. In this context, we have observed a decrease in TG stores in hearts treated with epinephrine, dibutyryl cAMP, and 3-isobutyl-1-methylxanthine. We attributed this effect to an increase in TG hydrolysis. Since TG turnover is rapid in muscle, it is necessary to measure TG synthesis and hydrolysis in the same heart. A unique aspect of the TG turnover work is that a TG lipase sensitive to cAMP will be measured (intracellular lipoprotein lipase). In the third study, an attempt will be made to determine if cAMP regulates intra-cellular TG metabolism in hearts of exercised rats. In the next study, we plan to determine if intracellular lipoprotein lipase (LPL) in skeletal muscle possesses the classical characteristics of LPL similar to that seen in heart. Further, we plan to determine if intracellular LPL in fast red, fast white, and soleus muscle is under hormonal control. Further, we plan to obtain information regarding enzyme-substrate interaction in skeletal muscle. The question here is whether intracellular LPL makes physical contact with the endogenous TG droplet. This work is important because little is known about LPL in skeletal muscle.